1. Field of the Invention
The present invention relates to techniques that enable the conversion of the scanning format of digital images, in particular converting from an interlaced format to a progressive format.
2. Description of the Related Art
The technological limits that accompanied the original development of television transmissions imposed the need for transmission of images in “interlaced” format, according to which the pixels of the odd and even lines (also referred to as top and bottom lines) of an image are scanned and transmitted alternately.
The majority of the visual display devices currently existing (in particular, cathode-ray-tube television apparatuses) support this type of scanning for the incoming video signal. The refresh frequency and the persistence of the image on the human eye for these apparatuses is in fact such that the alternation of the odd and even lines is not perceived by the human eye. Consequently, the television standards used practically throughout the world (PAL, SECAM, and NTSC) adopt in fact the interlaced format.
The appearance of new technologies of construction of visual displays which enable screen dimensions to be obtained that are larger than current ones (for instance plasma displays can reach 40″) has led to an increase in the quality of the information being displayed and accordingly an increase in the number of functions for processing the video signal.
Among the above functions one is precisely the conversion of scanning of the image from the interlaced format to the progressive format, which represents the ideal type of image for exploiting to the full the characteristics of the new visual displays.
Consequently, various categories of de-interlacing solutions have been developed, as evidenced, for example, by the following documents:
Pasi Pohjala, Matti Karlsson: Line rate upconversion in IDTV applications, IEEE Transactions on Consumer Electronics, Vol. 37, No. 3, pp. 309–312, August 1991;
Kai Oistamo, Yrjo Neuvo: A motion insensitive method for scan rate conversion and cross error cancellation, IEEE Transactions on Consumer Electronics, Vol. 37, No. 3, pp. 396–302, August 1991;
R. Simonetti, A. Polo Filisan, S. Carrato, G. Ramponi, G. Sicuranza: A deinterlacer for IQTV receivers and multimedia applications, IEEE Transactions on Consumer Electronics, Vol. 39, No. 3, pp. 234–240, August 1993;
H. Hwang, M. H. Lee, D. I. Song: Interlaced to progressive Scan Conversion with double smoothing, IEEE Transactions on Consumer Electronics, Vol. 39, No. 3, pp. 241–246, August 1993;
Dong-Ho Lee, Jong-Seok Park, Yung-Gil Kim: Video format conversion between HDTV Systems, IEEE Transactions on Consumer Electronics, Vol. 39, No. 3, pp. 219–224, August 1993;
Rob A. Beukeir, Imran A. Shah: Analysis of interlaced video signals and its applications, IEEE Transactions on Image Processing, Vol. 3, No. 5, pp. 501–512, September 1994;
P. Delogne, Laurent Cuvelier, Benoit Maison, Beatrice Van Caillie, Luc Vanderdorpe: Improved interpolation, motion estimation, and compensation for interlaced pictures, IEEE Transactions on Image Processing, Vol. 3, No. 5, pp. 482–491, September 1994;
Manfred Ernst: Motion compensated interpolation for advanced conversion and noise reduction, Proceedings 4th International Workshop on HDTV (Turin, Italy), Vol. 1, September 1991;
L. Capodiferro, A. Chiari, G. Marcone, S. Miceli: A screen format converter for HDTV, Proceedings 4th International Workshop on HDTV (Turin, Italy), Vol. 2, September 1991;
Bede Liu, Andre Zaccarin: A comparative study of coding of interlaced sequences with motion compensation, Proceedings 4th International Workshop on HDTV (Turin, Italy), Vol. 2, September 1991;
H. Blume, L. Schwoerer, K. Zygis: Subband based upconversion using complementary median filters, Proceedings International Workshop on HDTV (Turin, Italy), October 1994;
Dong Wook Kang: Two-channel spatial interpolation of images, Image Communication 16, pp. 395–399, 2000; and
Feng-Ming Wang, Dimitris Anastassiou, Arun Netravali: Time-recursive deinterlacing for IDTV and Pyramid Coding, Image Communication 2, pp. 365–374, 1990.
With a certain degree of simplification, but with substantial adherence to the actual situation, the above previously known solutions fit into four basic categories, namely:                spatial techniques;        temporal techniques;        spatial-temporal techniques; and        motion-compensated techniques.        
The various solutions differ a great deal both in terms of complexity and in terms of hardware resources required.
Spatial techniques use only the data coming from the same half-frame or field. It is recalled that in the technical terminology of the sector the term ‘frame’ is used to indicate the entire image, whilst the terms ‘half-frame’ or ‘field’ are used to indicate the interlaced image made up of just odd lines or even lines. A frame is thus made up of two fields, referred to as top field and bottom field. In progressive scanning, both of these field are transmitted together, whereas in interlaced scanning the two fields are transmitted alternately.
Temporal techniques use data coming from one or more fields, which are subsequent and/or previous to the current field in which the missing data to be reconstructed reside.
Spatial-temporal techniques use both types of data, i.e., both spatial ones and temporal ones.
Finally, motion-compensated techniques are based on both spatial-temporal data and on the estimation of the movement of the various parts of which the image is made up (calculation of the motion vectors). This motion information can be calculated externally and can hence be supplied from outside the system in question, or else may be estimated inside the conversion system.
In general terms, a greater number of available data enables a higher level of performance to be achieved. However, it may also happen that an incorrect use of this amount of data (for example, in the case of techniques based upon motion compensation, an incorrect estimation of the motion-field values) leads the system to diverge from the ideal solution, introducing artifacts such as to degrade the quality of the reconstructed image.